Cardiac muscle is rhythmyogenic; i.e., without any external electrical stimulus, the muscle cells of the heart will spontaneously self-excite and contract. However, without some form of pacing, the heart will enter into a state of fibrillation wherein the cardiac muscle cells contract rapidly in an irregular and unsynchronized manner. Fortunately, specialized pacemaker cells coordinate the beating of the heart. These pacemaker cells are collected at two nodes, the sinoatrial node and the atrioventricular node. The sinoatrial node spontaneously initiates a heartbeat by electrically stimulating muscle cells within the upper right atrium, which causes the right and left atrium to contract as the electrical impulses propagate through the surrounding cardiac tissue. When the electrical impulses reach the lower portion of the right atrium, the atrioventricular node then causes the ventricles to contract thereby completing a heartbeat.
A heart may beat incorrectly in several ways. That is, it may beat too quickly (tachycardia), too slowly (bradycardia), or the upper or lower chambers of the heart may contract in an unsynchronized, erratic manner (fibrillation). Each of these arrhythmias typically requires treatment; however, ventricular fibrillation requires immediate medical attention. To treat an arrhythmia, external electrical stimulation may be applied to the heart. Tachycardia, bradycardia, and atrial fibrillation may be addressed through cardioversion, which uses low level electrical pulses (or drug therapy) to return a heart to its normal rhythm. In contrast, ventricular fibrillation is treated with defibrillation wherein a more aggressive therapy is applied to the heart.
Implantable cardioverter defibrillator (ICDS) have been developed that may deliver low level electrical pulses to help pace a patient's heart and, if necessary, deliver high level electrical pulses to treat ventricular fibrillation. In general, an ICD comprises a pulse generator and a flexible lead assembly having a distal end and a proximal end coupled to the pulse generator. One or more pacing electrodes and one or more coiled defibrillation electrodes are disposed on the distal end of the lead assembly. When the pulse generator is implanted within a patient, the lead assembly is disposed proximate the patient's heart. If a transvenous ICD is employed, the distal end of the lead assembly is positioned within one or more chambers of the heart (endocardial lead), on the surface of the heart (epicardial lead), or within the surrounding vasculature. If a subcutaneous ICD is employed, the distal end of the lead assembly is positioned adjacent the heart.
An ICD should be capable of identifying and distinguishing between the different types of arrhythmias to determine the proper treatment to apply. To this end, ICDs may be equipped with one or more sense electrodes proximally disposed on the flexible lead assembly. Each of the sense electrodes register cardiac (or other bioelectric) signals and transmit these signals to control circuitry disposed within the ICD, which then determines if a particular arrhythmia has been detected. Since the sense electrodes are deployed on a flexible lead assembly, the position of the electrodes may move with respect to each and with respect to the pulse generator's conductive canister. This introduces a level of uncertainty in the detection of the cardiac signals. For example, the conductive canister may affect (e.g., attenuate) the bioelectric signals at the sense electrodes, which, in turn, makes accurate identification of arrhythmic conditions more difficult. If the sense electrodes are physically contacting the conductive canister, further interference with signal detection is possible.
Considering the above, it should be appreciated that it would be desirable to provide an implantable medical device (e.g., an ICD) wherein the relative positioning of the sense electrodes is maintained, and may be duplicated if desired. It would also be desirable to provide such a subcutaneous implantable cardiac sensing and/or treatment device wherein the sensing electrodes are maintained at least a predetermined distance from the system's conductive canister. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.